UC Riverside

Cowpea (Vigna unguiculata L. Walp) is an important food and fodder legume worldwide. Cowpea is grown extensively in sub-Saharan Africa where it is an important staple. Root-knot nematode (RKN, Meloidogyne spp.) is a significant pest in cowpea. Several sources of resistance to RKN have been identified in cowpea, including the widely used Rk gene. As part of a study to elucidate the mechanism of Rk -mediated resistance, the histological response to avirulent M. incognita feeding of a resistant cowpea cultivar CB46 was compared with a susceptible near-isogenic line. It was found that the Rk-mediated resistance is a delayed response which lacked an early hypersensitive reaction (HR). Typically in pathogen resistance HR is closely associated with an oxidative burst (OB) in infected tissue. Following a basal OB during early infection in both susceptible and resistant cowpea roots no significant OB was detected, a profile consistent with the histological observations of a delayed resistance response.

As cowpea does not have a microarray platform, a readily available soybean (Glycine max) genome array was used to identify single feature polymorphisms (SFP) in cowpea. An algorithm called Robustified Projection Pursuit was used for data analysis and RNA was used as a surrogate for DNA to detect SFPs between two genetically diverse cowpea genotypes. Cowpea genomic amplicon sequencing was performed for SFP validation and a 71% validation rate was observed. This study demonstrated that the soybean genome array is a satisfactory platform for identification of SFPs in cowpea. Furthermore, this study provided an example of a method to extend genomic resources from well supported species to "orphan" crops like cowpea.

Transcriptome profiles of incompatible and compatible cowpea-RKN interactions for two time points using the Affymetrix soybean GeneChip were generated. The profiles indicated that the RKN resistance pathway was still partially suppressed at 9 days post-inoculation in resistant cowpea roots. Subtle variation of ROS concentration, induction of toxins and other defense related genes were indicated to play a role in this unique resistance mechanism. Further functional analysis of these differentially expressed genes will advance understanding of this intriguing plant-nematode interaction in a more precise manner.